(1) Field of the Invention
The invention relates to the general field of vacuum enclosures, more particularly to flat panel displays and methods for sealing in the vacuum.
(2) Description of the Prior Art
A considerable body of art already exists with respect to ways to seal a permanent vacuum inside a glass or metal enclosure. Most widely used is the tipping off method wherein the glass enclosure includes a tube that is an integral part of it, being either cast or blown in place during manufacture. This tube is then used as a vacuum port through which the interior of the structure is evacuated. Once the requisite level of vacuum has been achieved, the tube is heated over a small annular area well removed from the main enclosure. This heat treatment serves to soften the glass which, under the influence of atmospheric pressure, then slowly collapses until the vacuum has been sealed in place. The heat is then immediately removed to prevent further collapse and any tubing that extends beyond the seal is removed.
There are several problems associated with the tipping off method outlined above. Even though a good vacuum may have been achieved just prior to tip-off, the softening of the glass results in considerable outgassing, just as evacuation ceases, so the final permanent vacuum inside the enclosure is only of the order of 10.sup.-4 torr. In order to achieve an acceptable level of vacuum it becomes necessary to make use of a gettering procedure. While usually effective, gettering adds significantly to the overall cost of manufacturing the product.
Another problem associated with tip-off is that the glass material used is fragile. Aside from being susceptible to mechanical breakage during its life, there is also the possibility that the tip-off tube may crack during sealing if the heat is not uniformly applied. Finally, heaters that generate temperatures around 1,000.degree. C. need to be provided for tip-off to work reliably in a production environment. Such heaters, temperature cycled many times a day, have a relatively short operating life, further adding to the manufacturing costs.
A different approach to the problem of sealing in a permanent vacuum is pinch-off. Here, the tube that is used for the vacuum port is made of metal. Once the requisite vacuum level has been achieved, the metal tube is pinched off using a suitable crimping tool. The latter is a pair of parallel jaws that press in on the tube from opposite sides. The metal is soft enough so that it deforms without cracking until the inner surfaces (of the tubing) that are being pushed together meet and then fuse as a cold weld. Provided the jaws remain parallel while the tube deforms and the inside surface of the tubing is free of grease, oxide, etc., the weld that is formed provides an effective vacuum tight seal. Any excess metal tubing is then removed.
The pinch-off method enjoys several advantages over tip-off. In particular, since no heating is involved there is no danger to the nearby glass enclosure that has been evacuated and, since outgassing is avoided, gettering is not usually needed, although it may still be used if an exceptionally high vacuum is to be maintained over a long period of time. The main problem with using pinch-off is how to achieve a reliable, low cost connection between the metal tubing that is to be pinched-off and the glass enclosure itself. In the current art, such glass to metal unions are formed by glass-to-metal seals or by housekeeper seals. These are limited to a small subset of both metal and glass types. In general, the metals are limited to OFHC copper, stainless steel 304, nickel, and gold, while the glass has been limited to Corning codes 7740 and 7052FN. A wider selection of glasses and metals has not been used because of thermal mismatch problems as well as cost.
A number of references relating to this problem can be found but none, as will be seen, anticipates the present invention. Akimoto et al. (U.S. Pat. No. 5,336,287 August 1994) describe a method for bonding metal to glass. A glass frit is used but the key feature of the invention is the inclusion of an aluminum borate whisker in the frit mix. The metal that is used is in the form of a stud that is to be bonded to the glass surface. The relative expansion coefficients of glass, metal, and frit are all carefully controlled.
Glass frit is also used by Tsukui et al. (U.S. Pat. No. 5,304,890 April 1994) to weld a reinforcing frame to a cathode ray tube while Schoene (U.S. Pat. No. 4,713,580 December 1987) teaches how a seal button may be used as part of assembling a metal vapor arc discharge lamp.